Yep, infact during WW1 a Celtic player was awarded the Victoria Cross, and a lot of those players went away to fight, as did several in WW2. As stated above though, it's a certain section of fans, who are mainly from Ireland, or youngsters who love that sort of crap
Some of you may not have seen this it was in the Irish times a few years back.
Gaelic Athletic Association Rule 42 stated that no Foreign Games could be played at Croke Park in Dublin, by which they meant football (soccer)
Here we have some idiot in a celtic tracksuit outside croke park holding a placard saying "no to foreign games"